![]() ![]() This is particularly important in hypothermia, where an increase in blood viscosity will cause problems with blood circulation. Because of that, blood behaves as a non-Newtonian fluid. Other factors influencing blood viscosity include temperature, where an increase in temperature results in a decrease in viscosity. This will lead to decreased oxygen delivery 7. When the hematocrit rises to 60 or 70%, which it often does in polycythemia rubra vera 6, the blood viscosity can become as great as 10 times that of water, and its flow through blood vessels is greatly retarded because of increased resistance to flow 2. This relationship becomes increasingly sensitive as hematocrit increases. One unit increase in hematocrit can cause up to a 4% increase in blood viscosity 4. Nevertheless, hematocrit has the strongest impact on whole blood viscosity. Plasma’s viscosity is determined by water-content and macromolecular components, so these factors that affect blood viscosity are the plasma protein concentration and types of proteins in the plasma 5. Red blood cells have unique mechanical behavior, which can be discussed under the terms erythrocyte deformability and erythrocyte aggregation 4. This leads to entrapment of red cells and clot formation, which dramatically increases blood viscosity. If clotting mechanisms are stimulated in the blood, platelet aggregation and interactions with plasma proteins occur. When whole body hypothermia is induced in critical care or surgical situations, this leads to an increase in blood viscosity and impaired organ blood flow. However, if a person’s hand is exposed to a cold environment and the fingers become cold, the blood temperature in the fingers falls and viscosity increases, which together with sympathetic-mediated vasoconstriction decreases blood flow in the cooled region. Normally, blood temperature does not change much in the body. Viscosity increases about 2% for each degree centigrade decrease in temperature. Therefore, there is an inverse relationship between temperature and viscosity. Just like molasses, when blood gets cold, it becomes “thicker” and flows more slowly. Some patients with anemia have low hematocrits, and therefore reduced blood viscosities.Īnother important factor that influences blood viscosity is temperature. Increased viscosity increases the resistance to blood flow and thereby increases the work of the heart and impairs organ perfusion. In fact, increasing the hematocrit from 40 to 60% (a 50% increase) increases the relative viscosity from 4 to 8 (a 100% increase). Patients with an abnormal elevation in red cell hematocrit (polycythemia) have much higher blood viscosities. At a normal hematocrit of 40%, the relative viscosity of blood is about 4. Therefore, blood viscosity strongly depends on hematocrit. Note that the increase is non-linear increased hematocrit causes a disproportionate increase in relative viscosity. Increasing red cell hematocrit increases relative viscosity. In the figure, the relative viscosity at 0% hematocrit (plasma without cells) is about 1.8 as shown by the y-intercept. ![]() Of these formed elements, red cells have the greatest effect on viscosity. The addition of formed elements to plasma (red cells, white cells, and platelets) further increases the viscosity. In fact, plasma at 37☌ is about 1.8-times more viscous than water at the same temperature therefore, the relative viscosity (ηr) of plasma compared to water is about 1.8 3. Because of molecular interactions between these different components of plasma, it is not surprising that plasma has a higher viscosity than water. Whole blood viscosity is the sum of plasma viscosity (mainly determined by colloid components such as albumin) plus the density and packing of all blood cells and their rigidity (see Figure 1) 2. Blood viscosity is determined by plasma viscosity, hematocrit (volume fraction of red blood cell, which constitute 99.9% of the cellular elements), red blood cell deformability, mechanical properties of red blood cells, red blood cell aggregation, and plasma viscosity.Īlthough plasma is mostly water, it also contains other molecules such as electrolytes, proteins (especially albumin and fibrinogen), and other macromolecules. Normal serum viscosity relative to water is 1.4 to 1.8 cp 1. Viscosity is measured in the unit of centipoise (cp). Viscous blood are thicker and travel more slowly. When blood has low viscosity, it travels quickly and without much difficulty. Viscosity is formally defined as the measurement of the internal resistance of a fluid to flow but can simply be thought of as the “thickness” or “stickiness” of a fluid. Blood viscosity is the thickness or stickiness of blood. ![]()
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